This invention relates to sorting apparatus. It is particularly concerned with such apparatus which grades product according to colour characteristics, and activates an ejection mechanism based on that grading to remove selected product from the stream. The present invention is directed at an optical system for monitoring light at a viewing station in sorting apparatus in order to grade product passing therethrough.
Product can be effectively graded by a colour sorting technique. Various sorting apparatus which grade product according to its ability to reflect light in different wavelength ranges are described in U.S. Pat. Nos. 4,203,522; 4,513,868; 4,630,736; 4,699,273; and 5,538,142, the disclosures of which are incorporated herein by reference. In apparatus disclosed in the '522 and '142 patents for example detectors are responsive to light reflected from a product in different wavelength ranges, and generate signals indicative of different qualities of the product. These signals are compared and analysed, to generate a signal which can activate ejectors to remove the relevant item from the product stream.
In some of the apparatus of the kind described above, the reflected light is monitored by optical systems containing CCD arrays with a plurality of lines of sensing elements. Typically a tri-linear array is used; the three lines of elements view different areas of the product and are filtered to respond to particular wavelength ranges. In order that the colour of an area of the product may accurately be determined it is necessary to compare measurements taken on the three lines of elements at different times. This may be achieved if the speed of the product is constant and is known accurately. However, in practice the speed of the product may vary, the product may move across the stream and it may rotate between sensing positions all of which give rise to difficulties in determining the colour. To avoid the problem it is necessary that the lines of arrays all view the same area of the product simultaneously; i.e. their view is co-incident. Previously this has been addressed by others as detailed in U.S. Pat. No. 5,315,384, by building cameras which split a beam of focussed light from a viewing area into a plurality of paths by use of an arrangement of prisms. The selection of colours in the beams is by filters which are cemented together with the prisms and the arrays. The positioning of the components must be very accurate which makes production of these cameras difficult and expensive, and major colour changes cannot be made to a camera. The introduction of multi-linear CCD arrays offered the possibility of simpler assembly and interchangeability of filters if the problem of the absence of co-incident viewing could be addressed.